Environmental Engineering Reference
In-Depth Information
13, compared with 7.3 for cement waste forms. 12 This represents an
improvement of almost six orders of magnitude in the effective diffusion
coefficient over cement. Irradiation to 10 8 rad and thermal cycling of
bitumen waste forms can increase leachability by up to 1.5 orders of
magnitude, but releases are still low with respect to cement waste forms
that do not use special sorptive agents to retard 137 Cs release.
Process limitations for bituminization of wastes include:
Flammability
Poor thermal conductivity
Relatively low compressive and tensile strength
Potential radiation damage of the organic matrix that can limit the quan-
tity/activity of the treated waste
Fires and explosions in bitumen processing systems are relatively common and
have significantly limited the application of this technology worldwide. This problem
is exacerbated by the presence of nitrate and other oxidizing agents in the waste,
which can lower the flash point of bitumen and accelerate combustion. 14 For example,
a fire and ensuing explosion occurred at the Low-Level Radioactive Waste Bitumi-
nization Facility installed at the Tokai Works Nuclear Fuel Reprocessing Plant in
Japan, where nitrate-based aqueous concentrates were being encapsulated in bitu-
men. 15 Poor thermal conductivity requires large heat transfer surfaces and limits the
throughput of the process.
The semisolid nature of bitumen results in low strengths and creeping of the
material under stress or unconfined compression. It will slowly flow over time under
its own weight to the lowest contained point, hence, the use of aggregate or gravel
for asphalt to provide structural stability while the asphalt fills the interstitial space
to bind and seal the composite. Low compressive strength (0.9 MPa, or 130 psi) 12
is indicative of poor mechanical integrity and can impact long-term durability. Thus,
secondary containment may be required for acceptable performance under landfill
disposal conditions. 16 Low tensile strength results in significant swelling of solidified
salt waste concentrates when subjected to saturated conditions as the salts rehydrate.
In testing at Brookhaven National Laboratory (BNL), bitumen waste forms contain-
ing soluble sulfate and borate salts swelled to almost ten times their original size
following long-term immersion in water. The swelling results from the osmotic
pressure exerted as water diffuses through the organic to the salt crystals, first
dissolving then diluting the salt as concentrated pockets of salty solution grow around
the encapsulated soluble salts. The bitumen creeps under the force of this osmotic
pressure, allowing these pockets to grow in size. The driving force is the difference
in salt concentration between the bulk immersion solution and these pockets. In
general, the organic layer thins, then breaks, allowing a physical pathway to equalize
concentrations/pressure, before the concentrations can equilibrate by diffusion
through the organic to slow and stop the growth of these pockets.
Radiation damage in bitumen has been shown to be quite low in total doses up
to the U.S. Nuclear Regulatory Commission-recommended 10 8 rad, but increases
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